1. Field of the Invention
The present invention relates generally to a method and system for increasing the holographic storage capacity of a holographic recording medium using an irradiance-tailoring element by changing the irradiance profile of the modulated data beam to thereby minimize the effects of fixed-pattern noise buildup from occurring in a holographic recording medium. The present invention further relates generally to a method and system for changing the irradiance profile of a relayed modulated data beam to thereby minimize the effects of fixed-pattern noise buildup in the holographic recording medium
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Holographic systems typically involve the storage and readout of entire pages of information, these pages consisting of arrayed patterns representing information. In general, a holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium.
Holographic systems are characterized by their high density storage potential and the potential speed at which the stored information is randomly accessed and retrieved. In fact, because information is typically manipulated, i.e., stored and retrieved, on a page-by-page basis, the speed of storage and retrieval compares favorably to conventional magnetic disk or compact disk storage systems. A significant advantage of holographic systems, however, is storage capacity. It is possible for each page stored as a holographic image to contain thousands or even millions of elements. Theoretically, it is believed that at the present time, up to 1014 bits of information are storable in approximately 1.0 cm3 of holographic storage medium.
The page-wise systems of holographic systems involve the storage and readout of an entire two-dimensional representation, e.g., a page of data Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in Psaltis et al., “Holographic Memories,” Scientific American, November 1995.
The capabilities of typical holographic recording systems are determined in part by the storage medium. One type of holographic recording media used recently for such systems are photosensitive polymer films. See, e.g., Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, (Los Angeles, Calif., 1990), pp. 1212-03. The holographic recording media described in Smothers et al., supra contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing (recording) of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting diffusion create a refractive index change, thus forming the holographic grating (hologram) representing the data.
Photosensitive polymer films are considered attractive recording media candidates for high density holographic data storage. These films have a relatively low cost, are easily processed and can be designed to have large index contrasts with high photosensitivity. These films can also be fabricated with the dynamic range, media thickness, optical quality and dimensional stability required for high density applications. See L. Dhar et al., “Recording Media That Exhibit High Dynamic Range for Holographic Storage,” Optics Letters, 24, (1999): pp. 487 et seq. seq.
A technique for increasing data storage capacity is multiplexing holograms. Multiplexing holograms involves storing multiple holograms in a media, often in the same volume or nearly the same volume of the media. Typically, this is carried out by varying an angle, wavelength, phase code, or some other system parameter in the recording and readout setup of the holograms. Many of these methods rely on a holographic phenomenon known as the Bragg effect to separate the holograms even though they are physically located within the same volume of media. Other multiplexing methods such as shift and, to some extent correlation, use the Bragg effect and relative motion of the media and input laser beams to overlap multiple holograms in the same volume of the media.
At least two fundamental factors typically govern the amount of data that can be holographically stored in a given volume of the holographic recording medium. First, the volume of the holographic recording medium required for the data and reference signal beams to interfere determines the number of discrete optical recording sectors allowed across the surface of a given area of the medium. Second, the number of data images that can be stored in a single volumetric sector of the holographic recording medium determines the local density of the data. Thus, in order to maximize the data capacity for any volumetric sector of the holographic recording medium, the combined volume of the object and reference beams should be minimized while the number of holograms is maximized.
One method available to reduce the volume of the holographic recording medium exposed during recording is to use a strong lens (e.g. Fourier Transform lens) to focus the modulated data beam into the center of the thickness or volume of the recording medium. This method enables equal and minimally sized irradiance profiles to be incident upon both surfaces of the medium. The undesired side-effect of using a focused beam centered in the volume of the holographic recording medium is that local areas of extremely high energy irradiance are created, which in turn over-expose and distort the recording medium, to the detriment of high-fidelity holographic recording. Such strong intensity or irradiance concentrations, the most detrimental of which are often localized along the center of the focused beam, have particularly negative implications for holographic recording approaches that multiplex many holograms in the same spatial location, such as angular multiplexing or correlation multiplexing.
Methods exist to mitigate these undesired effects that occur as a result of focusing the object beam into the center of the volume of the holographic recording medium. These methods include shift multiplexing, reducing the number of multiplexed holograms, reducing the energy in the data beam, or using a stationary or fixed “phase mask” to minimize the coherent addition of pixels at the Fourier plane of the focused data beam. See, for example, U.S. Pat. No. 5,510,912 (Blaum et al.), issued Apr. 23, 1996, and U.S. Pat. No. 5,727,226 (Blaum et al.), issued Mar. 10, 1998, which use stationary or fixed “phase masks.” All of these methods may negatively and dramatically impact the available storage capacity of the holographic recording medium.
What may be the most dramatic example of a non-uniform irradiance distribution occurs when a holographic storage system uses an amplitude (data) modulator to modulate data onto the optical beam, with the hologram then being recorded in the Fourier plane of the modulator. A Fourier plane recording occurs when there is a lens that focuses the information from the data modulator into the recording medium. In this particular instance, every pixel adds coherently at the very center of the Fourier distribution, thus often leading to what is commonly referred to as a “Direct Current (DC) hot spot.” The irradiance (intensity) of this DC hot spot may be as much as six orders of magnitude higher than the surrounding data. The focused portion of the data beam, known as the beam waist, represents the smallest area of the focused beam that may be achieved, and is therefore the data beam location where the highest bit density often occurs. The focal length of the lens and the pixel size determine the smallest area of the focused beam that may be achieved, as defined by the equation D=2πλf/Δ, where D is the diameter of the beam waist at the focus, λ is the wavelength of the beam, f is the focal length of the lens, and Δ is the pixel diameter.
There have been very few methods developed to decrease this DC hot spot effect. The most common method uses a “phase mask” on the modulator that scrambles the phase of the pixels so that they do not constructively add at the center of the Fourier plane. Common variations of a phase mask may include binary pixel matched phase masks, multilevel pixel matched phase masks, and axicons. See Psaltis et al., Holographic Data Storage (2000), for a summary of research on these types of phase masks. Because of the difficulty in aligning a pixel matched phase mask to an amplitude (data) modulator, the practicality of using such a phase mask is questionable.
Another approach uses a phase shifting element composed of “linear variations” to provide a non-pixel matched solution to this problem. See U.S. Pat. No. 6,281,993 (Bernal et al.), issued Aug. 28, 2001. This phase shifting element includes an axicon or a “phase shifting device that consists of a plurality of prismatic elements. These prismatic elements have linear features, and as such, these elements contain discontinuities. These discontinuities are difficult to manufacture and may also result in a potential loss of data when the discontinuity occurs in the middle of a pixel. Where an axicon is used, there is only one discontinuity, minimizing the effect. Instead, the axicon causes the DC component of the irradiance profile to become donut shaped, which is not an ideal way to smooth the irradiance profile in the focus plane.
Continuous random phase masks disclosed in D. G. Esaev, Continuous Random Phase Mask,” Sov. Phys. Tech. Phys., 1977, 9, 1150-1152, involving continuous random phase pixels may accomplish smoothing of the irradiance profile in the focus plane. See also A. Iwamoto, “Artificial Diffuser for Fourier Transform Hologram recording, ” Appl. Optics, 1980, 19, 215-220, which discloses a phase mask of random pixels but where the phase variation is of a higher spatial frequency (features smaller than the pixels) than the data pixels and where the phase variation is continuous and smooth rather than having discrete steps. However, the phase pixels in both these masks have the same alignment, registration and interference difficulties as the two and multiple level masks.
Continuously varying phase masks may be used in a holographic data storage system to create a similar effect to that of pixel matched phase masks without the strict alignment requirement, and enabling imaging onto the data modulator without the need to match pixels. However, one detrimental effect in using these phase masks in holographic recording is that, for a fixed position of the phase mask location, the irradiance profile of the beam passing through the phase mask still has a constant pattern, even though the irradiance profile of the data modulator is relatively random. When relayed or focused into holographic media with a lens (or lenses), a patterned irradiance distribution may build up an index profile in the media because the media is inherently sensitive to variations in irradiance. This buildup may lead to non-uniformities, scatter, media distortions, and other detrimental effects.
These detrimental irradiance concentration effects also vary depending on the optical characteristics of the holographic recording system, such as the aberrations present in the optical system, aberrations induced by the incident angle between the recording beams and the recording medium, and intensity non-uniformities in the illumination beams. The precise irradiance profiles may also vary based on system design, but the repeated exposure of any static irradiance profile, even when the data beam is modulated, will build up a refractive index profile which is the autocorrelation of the static irradiance profile. This undesired refractive index profile ultimately results in the degradation of the recording fidelity of the medium.
Accordingly, it would be desirable to provide a holographic recording method and/or system that is able to: (1) minimize the recording medium volume required for the object and reference beams to interfere; (2) maximize the data capacity in any volumetric sector of the recording medium; (3) avoid or minimize over-exposure, distortion and other detrimental effects in the holographic recording medium, even when multiplexing many holograms in the same spatial location; (4) maintain a close ratio in power density between the object and reference beams; and/or (5) minimize the alignment tolerance issues of phase masks.